Rapid wave-driven advective pore water exchange in a permeable coastal sediment
In: Journal of Sea Research. Elsevier/Netherlands Institute for Sea Research: Amsterdam; Den Burg. ISSN 1385-1101; e-ISSN 1873-1414, more
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| Keywords |
Measurement > Flow measurement
Oscillatory flow
Particulates > Suspended particulate matter
Properties > Physical properties > Permeability
Sedimentary structures > Bed forms > Sand ripples
Sediments
Transport processes > Advection
Transport processes > Diffusion
Water > Pore water
Water waves > Gravity waves
Water waves > Surface water waves
MED, Italy, Tuscany, Giglia I. [Marine Regions]
Marine/Coastal |
| Author keywords |
permeable sediment; coastal sand; advection; in-situ optode; pore waterflow velocity measurements; surface gravity waves |
| Abstract |
In this study we present in-situ measurements of pore water flow velocities in a coastal sandy sediment (permeability = 3.65×10-10 m2). The advective pore water flows were driven by the interaction of oscillating boundary flows with sediment wave ripples, (amplitude = 7 cm, wavelength = 30 to 50 cm). The measurements were carried out in the Mediterranean Sea at 50 to 70 cm water depth during a phase of very low wave energy (max. wave applitude = 10 cm). An optode technique is introduced that permits direct pore water flow measurements using a fluorescent tracer. Near the sediment surface (0.5 cm depth) pore water reached velocities exceeding 40 cm h-1. Thus, advective transport exceeded transport by molecular diffusion by at least 3 orders of magnitude. Based on the pore water velocity measurements and ripple spacing, we calculate that 140 L m-2 d-1 are filtered through the sediment. Pore water visualisation experiments revealed a flow field with intrusion of water in the ripple troughs and pore water release at the ripple crests. The wave-driven water flow through the sediment, thus, was directly linked to the wave-generated sediment topography, and its spatial dimensions. These results show that surface waves cause water filtration through permeable sediments at water depths smaller than half the wavelength. We conclude that surface gravity waves constitute an important hydromechanical process that may convert large areas of the continental shelves into expansive filter systems. Surface gravity waves thereby could affect suspended particle concentration and cycling of matter in the shelf. |
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